1. Field of the Invention
The present invention relates to a semiconductor device which is capable of data communication (reception/sending) through wireless communication.
2. Description of the Related Art
Recently, an individual identification technique using wireless communication (hereinafter referred to as a wireless communication system) has attracted attention. Specifically, RFID tags (also referred to as IC tags, IC chips, RF tags, wireless tags, or electronic tags) have begun to be used for production, management, and the like of individual objects. In addition, the RFID tags are expected to be used for personal authentification.
The wireless communication system is a system which performs data communication using radio signals between a power supply-cum-sender/receiver or a sender/receiver such as a communication device (also referred to as an interrogator or a reader/writer), and a sender/receiver such as an RFID tag (hereinafter referred to as an RFID tag).
As for the wireless communication system, research and development for improving efficiency of power supply from a communication device to an RFID tag have been actively conducted in order to extend a communication distance between the communication device and the RFID tag.
In general, electric power is in inversely proportional to the square of the distance between a spot where the electric power is emitted and a spot where the electric power is observed. Therefore, the longer the distance (hereinafter referred to as a communication distance) between the communication device and the RFID tag becomes, the less electric power the RFID tag receives, whereas the shorter the communication distance becomes, the more electric power the RFID tag receives.
When the communication distance is long and the RFID tag receives a small amount of electric power, power consumption of the RFID tag can be reduced as one of methods for performing normal communication. For reduction of power consumption of the RFID tag, a transistor which can drive at a low voltage has been increasingly used.
On the other hand, when the communication distance is short and the RFID tag receives a huge amount of electric power, Patent Document 1 (Japanese Published Patent Application No. 2002-368647) can be used as one of methods for performing normal communication.